1. Field of the Invention
The present invention relates to an integrated optical circuit (also Integrated Optic Circuit) comprising a generally planar substrate and at least one optical waveguide. More precisely, the invention relates to an integrated optical circuit comprising means for attenuating the propagation of spurious optical waves via the substrate.
2. Description of the Related Art
The fabrication of integrated optical circuits is based on the use of microlithography techniques that allow a series production. A single-mode optical waveguide may be fabricated on a planar substrate by steps of masking and deposition of a narrow strip of material, possibly followed by a step of thermal diffusion. In an integrated optical circuit, as in an optical fiber, the optical guiding effect is linked to a difference of refractive index between the optical waveguide and the substrate, the refractive index of the waveguide being higher than that of the substrate. Different materials may be used for the fabrication of integrated optical circuits, such as III-V semiconductors, silica on silicon, glass, or also lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). Lithium tantalate and lithium niobate are particularly interesting materials because they have a Pockels electro-optical effect. By placing electrodes on either side of the integrated waveguide, it is possible to modulate the waveguide index and then to modulate the phase of an optical signal propagating in the waveguide. In an integrated optical circuit where the electrodes are separated by about ten micrometers, the application of a voltage of only a few volts is sufficient to generate an electrical field and to induce the desired phase modulation. By way of comparison, in a conventional optic phase modulator, the electrodes being separated by at least one millimeter, the electric voltage required to generate a same electric field between the electrodes is of several hundreds of volts.
Different technologies of fabrication of integrated optical circuits on lithium niobate have been developed, firstly the titanium (Ti) diffusion technique, then the proton-exchange technique. The titanium diffusion technique consists in depositing a strip of titanium on the surface of a lithium niobate substrate, then to heat the substrate in such a manner that the titanium is diffused into the substrate and increases locally the refractive index. The titanium diffusion technique requires a high temperature (900-1100° C.). The proton-exchange technique consists in placing a birefringent LiNbO3 crystal in a bath of acid so as to replace Li+ ions by H+ ions (i.e. protons). The proton-exchange technique is performed at a lower temperature than the titanium diffusion. Moreover, the technique of proton exchange on a birefringent LiNbO3 crystal has for effect both to increase the extraordinary index of the crystal, which creates guidance for a polarization according to the extraordinary axis, and to reduce the ordinary index of the crystal, with the result that a polarization according to the ordinary axis is not guided. In the proton-exchange LiNbO3 circuits, the common configuration is a section X, the axis X of the single-axis birefringent LiNbO3 crystal being perpendicular to the surface of the substrate, whereas the axes Y and Z of the crystal are parallel to the surface. The axis of propagation of the waveguide is parallel to the direction Y, and the TE mode (where TE stands for “transverse electric”, i.e. an electric field parallel to the surface of the substrate) is parallel to the direction Z. In this case, the proton-exchange optical waveguide guides only the TE polarization state, the TM cross-polarization state (where TM stands for “transverse magnetic”, i.e. a magnetic field parallel to the surface of the substrate and thus an electric field perpendicular to the surface of the substrate) propagating freely in the substrate. The technique of proton exchange in lithium niobate hence allows to fabricate a polarizer on an integrated optical circuit.
Many integrated optical circuits are hence fabricated from lithium niobate: polarizer, phase modulator, Mach-Zehnder interferometer, Y-junction, 2×2 coupler or 3×3 coupler. Advantageously, a same optical circuit integrates several functions on a same substrate, which allows to improve the compactness and to reduce the optical connections. The integrated optical circuits obtained by lithium niobate proton-exchange find applications in particular in fiber optic gyroscopes.
In an integrated optical circuit, an input beam is generally coupled to an end of an optical waveguide through an optical fiber. However, only certain modes (for example, of polarization) are guided by the waveguide, the other modes propagating freely in the substrate. Moreover, if the core of the fiber is not perfectly aligned with the waveguide of the integrated optical circuit, a part of the incident light beam may be coupled in the substrate and may propagate outside the waveguide. A part of the light non guided by the waveguide may be reflected by total internal reflection on one or several faces of the substrate. In fine, a part of this non-guided light may be coupled to an output optical fiber facing to another end of the waveguide. The non-guided light may hence disturb the operation of an integrated optical circuit. For example, in the case of a lithium niobate proton-exchange polarizer, the polarization rejection rate may be affected by the coupling of the light transmitted in a non-guided manner by the substrate. Likewise, in the case of a 2×2 or 3×3 coupler, the non-guided light may be coupled via the substrate from an input to an output of the integrated optical circuit.
FIG. 1 schematically shows a perspective view of an integrated optical circuit according to the prior art. The integrated optical circuit comprises a planar substrate 10. By convention, in the present disclosure, the substrate 10 comprises an input face 1, an output face 2, a lower face 4, an upper face 3 and two lateral faces 5. The lower face 4 and the upper face 3 extend between the input face 1 and the output face 2. The lower face 4 and the upper face 3 are opposite to each other. Preferably, the lower face 4 and the upper face 3 are planar and parallel to each other. Likewise, the lateral faces 5 are planar and parallel to each other and extend between the input face 1 and the output face 2. The input 1 and output 2 faces of the substrate may also be planar and polished, but are preferably cut with an angle of inclination to avoid the spurious reflections between the ends of the waveguide. The substrate 10 comprises a rectilinear optical waveguide 6 that extends between a first end 7 on the input face 1 and a second end 8 on the output face 2. By convention, the waveguide 6 is nearer from the upper face 3 than from the lower face 4. According to a preferred embodiment, the optical waveguide 6 is located under the upper face 3 of the substrate and extends in a plane parallel to the upper face 3. The optical waveguide 6 may be delimited by the upper face or buried just under this upper face. In other embodiments, the waveguide 6 may be deposited on the upper surface 3 or may extend inside the substrate, for example in a plane parallel to the upper surface 3, half the way between the lower face 4 and the upper face 3. An input optical fiber 20 and an output optical fiber 30 are optically coupled to the first end 7 and respectively to the second end 8 of the waveguide 6. The input optical fiber 20 transmits an optical beam in the integrated optical circuit. A part of the optical beam is guided by the waveguide. The guided beam 12 propagates up to the end 8 of the waveguide 6 facing the output fiber 30. Due to a mode mismatching between the core of the optical fiber 20 and the integrated waveguide 6, another part of the beam is not coupled in the waveguide and propagates freely in the substrate 10. A non-guided beam 14 then propagates in the substrate up to the lower face 4 of the substrate. A part of the non-guided beam 14 may be reflected by total internal reflection on the lower face 4. A part of the reflected beam 16 may then be transmitted up to the end of the substrate facing the output fiber 30. The output fiber 30 may hence collect not only the guided optical beam 12, but also a part of the non-guided and reflected optical beam 16. FIG. 1 shows only a single reflection on the lower face 4 of the substrate, half the way between the input face 1 and the output face 2, i.e. at the centre of the lower face 4. Other multiple internal reflections are also possible.
FIG. 2 shows a sectional view of the integrated optical circuit of FIG. 1 in which has been schematically shown the angular distribution of the light power of the non-guided optical beam in the substrate. It is observed that a relatively high part of the optical beam is optically coupled in the substrate. The non-guided optical wave undergoes a total internal reflection on the upper surface 3. Therefore, the non-guided optical wave is subjected to an interferometric effect of the Lloyd mirror type on the upper face 3 of the substrate. This results in a Lloyd-mirror interferometer, interferences being produced between the input fiber 20 and its virtual image. Now, the total internal reflection produces a phase-shift of π. Consequently, the central fringe of the interferogram, located on the upper face 3, is a black fringe. This explains that the power density of the non-guided light propagating directly is drastically reduced just under the upper face 3, where is placed the output optical fiber (cf. H. Lefèvre, The fiber optic gyroscope, Artech House, 1992, Annex 3 Basics of Integrated Optics, pp. 273-284). Consequently, a proton-exchange polarizer would have in theory a very high polarization rate of −80 to −90 dB.
However, there exist other types of coupling of the non-guided optical beam than direct transmission. Indeed, the substrate may transmit different non-guided beams propagating by internal reflection, in particular on the lower face 4, but also on the upper face 3 or on the lateral faces 5. Non-guided spurious beams propagating by internal reflection on the faces of the substrate may reach the proximity of a waveguide end 8 on the output face 2 of the substrate.
Generally, the non-guided beams reflected inside the substrate may affect the quality of the signals transmitted in the waveguide of an integrated optical circuit. In the case of a lithium niobate proton-exchange polarizer, cut according to a plane X and comprising a waveguide integrated according to the axis of propagation Y, the guided beam 12 is generally a TE polarisation beam and the non-guided beam 14 is a TM polarisation beam. Due to the internal reflections of non-guided light in the substrate, the polarisation rejection rate of a proton-exchange polarizer according to the diagram of FIG. 1 is in practice limited to about −50 dB. Now, the quality of an integrated polarizer influences the performances of certain applications, in particular in a fiber optic gyroscope. It is hence necessary to improve the rejection rate of an integrated optical guide polarizer. More generally, it is desirable to improve the optical quality of an integrated optical circuit and to reduce the quantity of non-guided spurious light transmitted by the substrate outside the optical waveguide.
Different solutions have been proposed to solve the problem of spurious coupling of non-guided optical beams between a waveguide input and a waveguide output in an integrated optical circuit.
It is generally admitted that the main contribution to the spurious light comes from the primary reflection of a non-guided beam on the centre of the lower face 4 between a first waveguide end 7 on the input face 1 and a second waveguide end 8 on the output face 2. In order to eliminate the primary reflection on the lower face of a substrate 4, an integrated optical circuit comprising a central groove 25a arranged in the middle of the lower face 4 (cf. FIG. 3) has been developed. The central groove 25a extends over the whole width of the substrate according to a direction perpendicular to the direction of the waveguide 6. However, if a central groove 25a stops the non-guided beam 14a that is reflected at the centre of the lower face 4 of the substrate, it does not stop the multiple internal reflections produced between the lower face 4 and the upper face 3. FIG. 4 shows an example of a part of non-guided optical beam 14b propagating between a first waveguide end 7 and a second waveguide end 8, by double reflection on the lower face and simple reflection of the upper face to form a multiple-reflection spurious beam 16b. Therefore, a central groove on the lower face of the substrate allows to improve the rejection rate of a proton-exchange polarizer by several orders of magnitude, but the rejection rate remains limited in practice to about −65 dB.
The document EPI 111413 describes an IOC comprising at least one central groove extending over more than 70% of the thickness of the substrate and a lid to reinforce the structure so as to avoid a breaking of the IOC at the central groove.
In the case of a Y-junction, the U.S. Pat. No. 7,366,372 proposes to arrange a first central groove 25a on the lower face of the integrated optical circuit, half the way between the input face 1 and the output face 2, so as to eliminate the primary reflection, and a second central groove 25b on the upper face, arranged between the branches of the Y-junction, and half the way between the input face and the output face, so as to eliminate the part of the non-guided beam 14b propagating by multiple reflection in the substrate and reflecting on the middle of the upper face (see the sectional view in FIG. 5). However, the central groove 25b on the upper face 3 must not cut the waveguide 6 and is thus laterally limited so as not to cut the branches of the Y-junction. This solution is not generalizable to other types of integrated optical circuits.
The document EP1396741 describes an IOC comprising a waveguide formed in a thin layer sandwiched between two confinement layers, the three-layer structure being integrated on a substrate. The document EP1396741 also describes a groove extending in the thickness of the three layers and approaching the nearest possible to the waveguide to absorb the spurious light.